1. Field of the Invention:
The invention is in the field of optical elements based upon either inorganic and organic base elements. Such optical elements as windows, eyeglasses, goggles, faceshields, binoculars and optical instruments are known to be subject to fogging, that is the condensation of moisture on such optical surfaces when these surfaces are cooled below the temperature of the surrounding air. Since the heat transfer or conduction of inorganic glass optical elements is greater than that of an organic glass optical element, the tendency for fogging is generally greater on an inorganic glass optical element. However, under conditions of extreme humidity, even organic glass lenses are subject to fogging or moisture condensation on their surfaces.
2. Description of the Prior Art:
A review of prior art attempts to provide an antifogging substrate is provided in U.S. Pat. No. 3,700,487 wherein an antifogging coating is disclosed for such optical elements as glass, polycarbonate, polymethylmethacrylate, diethylene glycol bis allyl carbonate and copolymers thereof, methylmethacrylate and metal. The teaching of this patent includes a process for the preparation of the surface of the optical element substrate for subsequent coating so that adequate bonding of the antifogging coating can be obtained. Preparation of the surface is achieved by dipping the optical element into a solution of an alkyl titanate or by a hydrolytic surface treatment in which an alcoholic or aqueous caustic such as sodium or potassium hydroxide is applied to the optical element prior to coating with the antifog coating disclosed. Other methods of obtaining adequate adhesion of the antifog coating are disclosed as the application of a bonding coat of polyvinyl butyral or the application of a nylon lacquer sold under the trademark "Milvex 4000" manufactured by General Mills. The antifog coating which is subsequently applied, for instance, to a polycarbonate substrate, consists of polyvinyl alcohol in solution in combination with hydriodic acid as a cross-linking catalyst. The solution pH is approximately 2.3.
In. U.S. Pat. No. 3,484,157, there is disclosed an abrasion-resistant optical element having a directly-adhering, transparent surface coating comprising a vinyl polymer cross-linked with a dialdehyde cross-linking agent. Such dialdehydes as glyoxal, pyruvic aldehyde, 2 hydroxyadipaldehyde or glutaraldehyde can be used.